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Smoke from the Rim Fire is visible near the Hetch Hetchy reservoir on August 22, 2013.
Hot weather and dry brush have helped the Rim Fire grow fast and furious. Making matters worse, the blaze — like all large fires — has been creating its own weather patterns.
While creating weather sounds like something a James Bond villain would attempt, it's actually very common in fires.
The process stems from a very simple phenomenon.
"Hot air rises and cold air sinks," said Robert Balfour, a National Weather Service meteorologist.
As a fire burns, it creates a lot of hot air. That air rises, leaving behind a vacuum that sucks in more air, Balfour explained. This can create intense winds around the fire.
Imagine sticking your leg in a bathtub; as you pull it out, water rushes in from all sides to fill the void where your foot was. That's what happens as hot air lifts out of a fire and new air rushes in.
The next step occurs when this hot air reaches the cool heavens above.
When the smoke clouds rise to a certain height they start to freeze, forming what meteorologists call an "ice-cap."
"There's kind of a smooth lens-shaped cloud structure there," said Balfour.
Soon after, the cool air does what cool air likes to do: It sinks. This can create a serious backdraft as the air falls to the ground.
Picture someone throwing a bucket of water straight up in the air; as the liquid slams back to earth it will splatter in all directions. That's essentially what happens when the cooler air drops back into the fire.
"So that's like a river of wind if you will, hitting the ground and spreading out in all directions," said Balfour.
The video below shows a sped up version of this pattern from 2009's Station Fire.
The winds created during this kind of event can be very harsh.
Dick Fleishman, a firefighter and information officer with the U.S. Forest Service, says sometimes air flows reach 80 miles an hour and can change direction quickly.
"You'll have winds coming completely one direction and then literally a minute later it will be going completely the opposite direction," he explained. "And it will be going twice as hard."
For that reason, firefighters keep a constant watch on the weather. A special meteorologist is assigned to each fire to track it.
Smoke columns have been a daily event for the Rim Fire, said Fleishman. They usually form in the afternoon and taper off by evening. At one point there were five distinct columns visible at the same time across the blaze.
In addition to changing wind patters, some fires create thunderstorms, said the National Weather Service's Balfour.
When moisture is present in a fire, it can evaporate, forming heavy clouds that can turn into what's called a pyrocumulonimbus. These can cause lightning and rain — even hail. Sometimes the rain dampens the fire, but usually it evaporates before reaching the ground.
In rare cases, fire-induced wind patterns can even create a tornado. These are extremely dangerous, said Fleishman.
"Looks like a dust devil or a tornado, but it's all full of flames," he recalled of the times he has seen such an event.
Change of seasons
When a fire creates its own weather it often ends up spreading the damage even farther into a forested area.
While firefighters can contain these dangerous burns, Robert Balfour said there is usually only one way to stop them completely.
"It's called winter," he said.
Most fires, he said, get contained in a finite area and continue to smolder until the first snow fall extinguishes the flames.
That could be months away for the Rim Fire. For now, those fighting it are hoping to have full containment by Sept. 20. Then it will be up to nature to take its course.